The present invention relates to exhaust gas sensors. More particularly, the present invention relates to an oxygen sensor.
The automotive industry has used exhaust gas sensors in automotive vehicles for many years to sense the composition of exhaust gases, namely, oxygen. For example, a sensor is used to determine the exhaust gas content for alteration and optimization of the air to fuel ratio for combustion.
One type of sensor uses an ionically conductive solid electrolyte between porous electrodes. For oxygen, solid electrolyte sensors are used to measure oxygen activity differences between an unknown gas sample and a known gas sample. In the use of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas, (i.e., reference gas), is usually atmospheric air because the oxygen content in air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (xe2x80x9cemfxe2x80x9d) is developed between the electrodes according to the Nernst equation.
With the Nernst principle, chemical energy is converted into electromotive force. A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases (xe2x80x9cexhaust gas electrodexe2x80x9d), and a porous electrode exposed to a known gas"" partial pressure (xe2x80x9creference electrodexe2x80x9d). Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine""s exhaust. Also, a typical sensor has a ceramic heater attached to help maintain the sensor""s ionic conductivity. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:                               E          =                      xe2x80x83                    ⁢                                    (                                                -                  RT                                                  4                  ⁢                  F                                            )                        ⁢                          xe2x80x83                        ⁢                          ln              ⁡                              (                                                      P                                          O                      2                                        ref                                                        P                                          O                      2                                                                      )                                                    ⁢                  xe2x80x83                                        where        ⁢                  :                                        E        =                  xe2x80x83                ⁢                  electromotive          ⁢                      xe2x80x83                    ⁢          force                                        R        =                  xe2x80x83                ⁢                  universal          ⁢                      xe2x80x83                    ⁢          gas          ⁢                      xe2x80x83                    ⁢          constant                                                  F          =                      xe2x80x83                    ⁢                      Faraday            ⁢                          xe2x80x83                        ⁢            constant                          ⁢                  xe2x80x83                                        T        =                  xe2x80x83                ⁢                  absolute          ⁢                      xe2x80x83                    ⁢          temperature          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          the          ⁢                      xe2x80x83                    ⁢          gas                                                  P                      O            2                    ref                =                  xe2x80x83                ⁢                  oxygen          ⁢                      xe2x80x83                    ⁢          partial          ⁢                      xe2x80x83                    ⁢          pressure          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          the          ⁢                      xe2x80x83                    ⁢          reference          ⁢                      xe2x80x83                    ⁢          gas                                                  P                      O            2                          =                  xe2x80x83                ⁢                  oxygen          ⁢                      xe2x80x83                    ⁢          partial          ⁢                      xe2x80x83                    ⁢          pressure          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          the          ⁢                      xe2x80x83                    ⁢          exhaust          ⁢                      xe2x80x83                    ⁢          gas                    
Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (emf) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel-rich or fuel-lean, conditions without quantifying the actual air-to-fuel ratio of the exhaust mixture.
For example, an oxygen sensor, with a solid oxide electrolyte such as zirconia, measures the oxygen activity difference between an unknown gas and a known reference gas. Usually, the known reference gas is the atmosphere air while the unknown gas contains the oxygen with its equilibrium level to be determined. Typically, the sensor has a built in reference gas channel which connects the reference electrode to the ambient air. To avoid contamination of the reference air by the unknown gas, the sensor requires expensive sensor package that usually has complex features in order to provide sufficient gas sealing between the reference air and the unknown gas. Historically, these gas sealed sensor packages have demonstrated insufficient durability in the field. This problem can be avoided by using in-situ electrochemical oxygen pumping. In this method, the air reference electrode chamber is replaced by a sealed reference electrode with oxygen electrochemically pumped in from the exhaust gas. This method eliminates the exhaust gas contamination problem but creates its own drawbacks. That is, an expensive electronic circuit is required to do the electrochemical oxygen pumping.
What is needed in the art is a simplified gas sensor that employs an electrochemical pumping of oxygen.
The deficiencies of the above-discussed prior art are overcome or alleviated by the gas sensor and method of producing the same.
One embodiment of the sensor comprises an electrochemical cell having a solid electrolyte layer disposed between an exhaust gas electrode and a reference electrode. At least one protective layer is disposed in contact with the exhaust gas electrode with at least one via hole is disposed through the protective layer, and at least one reference gas channel is disposed in fluid communication with the reference electrode. Disposed in thermal communication with the electrochemical cell is a heater, with a resistor disposed in electrical communication with the heater and a pump electrode.
One embodiment of the method of using a gas sensor comprises disposing an electrochemical cell having a solid electrolyte between an exhaust gas electrode and a reference electrode. Disposing at least one protective layer in contact with the exhaust gas electrode at least one via hole through the protective layer, and at least one reference gas channel in fluid communication with the reference electrode. A heater is positioned in thermal communication with the electrochemical cell, and a resistor is disposed in electrical communication with the heater and a pump electrode and applying a voltage to the sensor.